This invention relates to an ultrasonic motor which frictionally drives a movable member by ultrasonic vibration using streching movement of a piezoelectric vibrator.
FIG. 28 shows a conventional ultrasonic motor.
A vibrating member 3 is fastened to a center shaft 9 fixed on a bed 7. A piezoelectric vibrator 4 is bonded to the bottom face of the vibrating member 3. The piezoelectric vibrator 4 has a plurarity of electrode patterns and is treated to be polarized. The piezoelectric vibrator 4 and a control circuit (not shown in the figure) are wired by a lead line 10. The projections 3a of the vibrating member 3 comes into contact with a movable member 2 through frictional materials 8. The movable member 2 is built into the center shaft 9 and can be rotatably supported by a bearing. The movable member 2 is urged downward by a pressure spring 1. When high frequency voltage is applied to the of piezoelectric vibrator 4, ultrasonic vibration is generated on the vibrating member 3 and the movable member 2 rotates by means of frictional force of the of frictional materials 8.
FIG. 29 is a perspective view showing an ultrasonic motor of a travelling wave type. A vibrating member 3 has a plurarity of projections at equal intervals. When a piezoelectric vibrator 4 is applied with a plurarity of high frequency voltage pulses having respectively different phases, a travelling wave is generated to the vibrating member 3, which rotates a movable member 2. Such a structure is disclosed in Japanese Kokai (Patent Provisional Publication) JP-A-No. 177874/1989.
FIG. 30 is a perspective view showing an ultrasonic motor of standing wave type. A vibrating member 3 has projections the number of which is relative to the number of crests of the standing wave. When one high frequency voltage is applied to a piezoelectric vibrator 4, a standing wave is generated to a vibrating member 3, which rotates a movable member 2. Such a structure is disclosed in Japanese Kokai JP-A-No. 107472/88.
As explained above, as to a piezoelectric vibrator of a conventional ultrasonic motor, an ultrasonic motor using a standing wave has only a piezoelectric vibrator which generates a standing wave in the a direction of a circumference of vibrating member. On the other hand, an ultrasonic motor using a travelling wave has only a piezoelectric vibrator which generates a travelling wave in the direction of the circumference the vibrating member. The rotation of the movable member is controlled by regulating voltage, application period, and frequency of a high frequency signal applied to the piezoelectric vibrator.
In both of the above cases, cross sections vertical to the direction of rotation of the movable members are uniform.
However, in order to perform accurate step drive such as for an analogue watch, a conventional ultrasonic motor requires rotation detecting means such as an encoder for reading the position of the movable member. This is because a feed of the movable member is not fixed when the application period of a high frequency signal is controlled simply by repeatedly switching on and off as an ultrasonic motor is frictionally driven. Therefore, in order to step-drive the movable member accurately, it is necessary to constantly monitor how much the movable member is moved so that a halt instruction can be given at the moment when the movable member reaches a desirable position. Accordingly, in the conventional ultrasonic motor, an accurate encorder is required, and there is a problem in that the movable member overruns a desirable position because of residual vibration of the vibrating member even after a halt instruction has been given.
Therefore, the object of this invention is to solve the conventional problems described above by obtaining an ultrasonic motor which accurately step-drives the movable member by simple operation without encoders.